1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system using the same, and in particular relates to a so-called lamination (multi-layer) type diffractive optical element in which a plurality of diffraction gratings made of different materials are laminated so that a high diffraction efficiency can be obtained in the entire region of use wavelength band.
2. Description of the Relating Art
A method for combining a plurality of lenses composed of glass materials respectively having different dispersions has been conventionally known as one of the methods for correcting chromatic aberration of a refractive optical system.
Also, as another method, methods for decreasing chromatic aberration by using a diffractive optical element having a diffractive action on a part of a lens surface or an optical system are disclosed in the literature of SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-open Patent Publication Nos. 1992-213421 (corresponding to U.S. Pat. No. 5,044,706) and 1994-324262 (corresponding to U.S. Pat. No. 5,790,321). These methods utilize a physical phenomenon in which chromatic aberration with respect to a certain reference wavelength appears in opposite directions at the refractive portion and diffraction portion in an optical system. Further, such a diffractive optical element is able to have an effect similar to that of an aspherical surface lens by adjusting the period of the periodic structure thereof, and with the diffractive optical element, various aberrations other than the chromatic aberration can be reduced.
Herein, in comparing the refractive action of light with the diffractive action thereof, on a lens surface having a common refractive action, although a single ray of light having a certain wavelength remains to be a single ray after being refracted, a single ray having a certain wavelength is divided into a plurality of rays having different diffraction orders on the diffractive surface.
Therefore, in a case where a diffractive optical element is used in an optical system, it is necessary to determine a grating structure so that a light flux of a use wavelength area is centralized at a specific diffraction order (which may be called a “design order”). In a case where light is centralized at a specific diffraction order, the intensity of light is made small at diffraction orders other than that. If the intensity of light is 0, no light of the diffraction order will exist. Therefore, it is necessary that the diffraction efficiency of the design order is sufficiently high to have the above-described features.
In view of such situations, the present applicant proposed a structure of preventing the diffraction efficiency from being lowered in a wide wavelength region in Japanese Laid-Open Patent Publication No. 1998-133149. The diffractive optical element proposed in the above is a lamination type diffractive optical element in which a diffraction grating 104 and a diffraction grating 105, made of different materials, are laminated on a substrate 102 as shown in FIG. 28. By respectively selecting appropriate values of the refractive indices of materials which compose two diffraction gratings 104 and 105, dispersion characteristics thereof, and grating thickness d1 and d2 thereof, a high diffraction efficiency can be achieved in the entire use wavelength region.
Also, a structure of preventing the diffraction efficiency from being lowered has been proposed in Japanese Laid-Open Patent Publication No. 1997-127322 (corresponding to U.S. Pat. No. 6,157,488). Herein, by optimally selecting three types of different materials and grating thickness d1 and d2 of three diffraction gratings 104, 105 and 106, and laminating them as shown in FIG. 29, a high diffraction efficiency is achieved in the entire visible region.
In the above-described prior art lamination type diffractive optical element, no description is given of the angles of diffraction grating side surfaces 104a and 105a (grating sidewall portions not contributing to image formation) shown in FIG. 28 and FIG. 29, and all of the grating sides 104a and 105a are made perpendicular to a surface Ha where the diffraction gratings are provided.
In a case where a diffractive optical element is used in an actual optical system, for example, an image-pickup optical system used for a silver film camera and digital camera, etc., and a part of an observation optical system such as a telescope, binoculars, microscope, etc., all the effective rays are not always made incident into diffraction gratings at any optional positions in the diffractive surface at the incident angle of 0° or at equal positive or negative angle distribution centering around 0°. Therefore, when the angles of the grating side surfaces at all the diffraction gratings are made perpendicular with respect to a surface where the diffraction gratings are provided, over the entire region in the diffractive surface, or when rays having an incident angle other than 0° or having an incident angle distribution center around an angle other than 0° are made incident, there are cases where the rays are eclipsed by the grating side surfaces, the ratio of rays not passing through an optical path to satisfy conditions for achieving a high diffraction efficiency is increased (in the present specification, these are commonly called “eclipse”), not only the quantity of effective light contributing to image formation is reduced, but also harmful light (light which adversely influences the image quality) which becomes a factor of flare, ghost, etc., is increased.